PLEISTOCENE EROSION OF TUK DALLES lil 



preglaciiil streams and the interglacial Saint Croix as the chief agents 

 of the valley sculpture. During the niaxinuun Kansan ghiciation the 

 ice-sheet here was douhtless very eflicient in planing down the rock 

 surface, anil it certainly aided to some degree in shaping the valley. It 

 also acted in the same way during the LUinoian and lowan stages. But 

 for some time in the closing Wisconsin stage of that later glaciation this 

 district, lying near the glacial boundary and its marginal moraines, was 

 less powerfully pressed and worn beneath the thin ice border, and in- 

 stead was characterized rather b}'' drift deposition. 



Outlet of the Western Superior Glacial Lake 



In the western part of the basin of lake Superior'the receding ice-sheet 

 lield a lake which outflowed southward through northwestern Wisconsin^ 

 across the present watershed, between the Bois Brule and Saint Croix 

 rivers. The highest shoreline of this lake at Duluth is 535 feet above 

 lake Superior (which has a mean level 602 feet above the sea) ; on mount 

 Josephine, about 130 miles northeast from Duluth, its height, according 

 to leveling by Doctor A. C. Lawson, is 607 feet; and at L'Anse and Mar- 

 quette, Michigan, 175 and 225 miles east of Duluth, it is found by iMr 

 F. B. Ta3dor about 590 feet above the lake. The northeastward uplift 

 averages seven inches per mile, and the eastward ascent is approximately 

 three inches per mile. 



The latest and lowest of the Western Superior beaches observed at 

 Duluth, occupied by the "boulevard " or pleasure driveway, 475 feet 

 above the lake, on the bluffs back of the cit}^ appears to have an ascent 

 of only about 35 feet in the distance to mount Josephine, showing that 

 the uplift of the land was quite rapidly in progress while the ice-front 

 still maintained the lake at the Saint Croix outlet. 



'Not long after the glacial retreat passed eastward beyond mount Joseph- 

 ine and Marquette, this lake was lowered and merged with lake Warren 

 across the lowlands of the northern })eninsula of Michigan. The vertical 

 interval between the final stage of the Western Superior lake and the 

 level of lake Warren shown b}^ its earliest beach at Duluth was about 

 60 feet. Thenceforward the outlet of lake Warren past Chicago carried 

 away the drainage from the glacial melting and rainfall of the Superior 

 basin. 



The old channel of outflow to the Saint Croix river has a width of 

 about a fifth of a mile in its narrowest place. Its bed is 1,070 feet above 

 the sea, or 468 feet above lake Superior, and it is bordered by bluffs 

 about 75 feet high, showing that when the course of outflow began here 

 the Western Superior glacial lake was about 550 feet above the present 



IV— Bull. Geol. Soc. Am., Vol. 12, 1900 



